Makefile.include: add $(MAKEFILEDIR) helper and use it
Makefile.include: add $(MAKEFILEDIR) helper and use it

$(MAKEFILEDIR) will always evaluate to the directory of the Makefile
where it's being called (evaluated).

This construct is used elsewhere in RIOT, make it more accessable
by defining a special variable with it.

OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,basilfx/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,basilfx/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,basilfx/RIOT,kYc0o/RIOT,basilfx/RIOT,kYc0o/RIOT,ant9000/RIOT